a hook allows tweaking tarball contents

this is a slightly different take on @jordimassaguerpla's
https://github.com/roman-neuhauser/bs-update/pull/2.
instead of a script, bs-update now calls a function, which
may be defined in .bs-update, or .bs-update-hooks, which
is sourced just before .bs-update.

two files can be used to provide shared defaults or keep
some configuration values private.